When conducting polymerizations on a commercial basis, it is important to utilize process conditions and components which will allow the molecular weight of the end products to be narrowly and reproducibly defined. The characteristics of a given polymer and its usefulness are dependent, among other things, upon its molecular weight. Hence, it is desirable to be able to predict with some certainty the molecular weight of the end product of the polymerization. When the molecular weight is not narrowly definable, or is not reproducible on a systematic basis, the process may not be commercially viable.
In the art, it is desirable to produce elastomeric compounds exhibiting reduced hysteresis characteristics. Such elastomers, when compounded to form articles such as tires, power belts and the like, will show an increase in rebound, a decrease in rolling resistance and will have less heat build-up when subjected to mechanical stresses.
A major source of hysteretic power loss has been established to be due to the section of the polymer chain from the last cross link of the vulcanizate to an end of the polymer chain. This free end cannot be involved in an efficient elastically recoverable process, and as a result, any energy transmitted to this section of the cured sample is lost as heat. It is known in the art that this type of heat loss can be reduced by preparing higher molecular weight polymers which necessarily have fewer end groups per given weight. However, this procedure is not always useful because processability of the rubber with compounding ingredients and during shaping operations decreases rapidly with increasing molecular weight.
U.S. Pat. No. 5,149,457 describes the use of mixtures of certain lithio amines to confer solubility in aromatic solvents. However, the use of aromatic solvents is often undesirable for anionic polymerizations. Moreover, the lithium amides or lihio amines described in that patent are either unsuitable or undesirable as anionic polymerization initiators, or will not give products with reduced hysteresis characteristics.
It is difficult to obtain consistent properties, such as a reduction in hysteresis characteristics, if the polymer cannot be controllably reproduced in a narrow molecular weight distribution range. See, for example, U.S. Pat. No. 4,935,471, in which some polymers are prepared with a heterogeneous mixture of certain secondary amines, including lithium pyrrolidide. Polymers made in this manner have widely varying molecular weights, broad polydispersities, and their functional terminations tend to be erratic, giving rise to poorly reproducible hysteresis reduction results.
A major drawback with many of these known anionic lithio amine initiators, is that they are not soluble in acyclic alkanes, such as hexane. Polar solvents have heretofore been employed including the polar organic compounds, especially ethers such as dimethyl or diethyl ether, as well as tetrahydrofuran, diethylene glycol methyl ether (diglyme).
The present invention provides novel, acyclic alkane soluble initiators, useful to initiate an anionic polymerization. The invention provides for the incorporation of two separate functionalities from the initiator at two or more ends of separate polymer chains. The invention also provides for efficient, controllable and reproducible polymerizations, with the preparation of well defined end-products of a relatively narrow molecular weight distribution range and having desired hysteretic properties.